The invention relates to a device and a process for drying solvent-containing plastic sheets or films.
Such solvent-containing plastic sheets or films result from the use of solvents particularly in the casting process. In such case, the dry casting process is the standard process for the production of sheets or films of cellulose acetate, polyamide, ethylcellulose, polyimide, polycarbonate, PVC, polyvinyl alcohol and other materials.
Low-boiling solvents are predominantly used to achieve a sheet formation by evaporation of the solvent in as short a time as possible with the dry casting process. The sheets are produced on casting machines. From the casting vessel the solution is brought to a metal conveyor through the nozzle of the caster. The predried film is removed by the conveyor and passes through a drying oven by a path as long as possible with correspondingly numerous freewheeling undriven carrier rollrs. The remaining solvent is then removed. The sheet is pulled through the drying oven by a driven roller and then wound into a roll by a winding machine.
To avoid the formation of folds, especially in the case of thin sheets, the carrier rollers are arranged so that the sheets are carried over numerous rollers with short, free paths and deflected. These freewheeling carrier or reversing rollers are placed inside of the drying oven. Problems occur in the drying of thin sheets. The longitudinal tension increasing from roller to roller as a result of bearing friction results in a transverse contraction with formation of undulations between the rollers. This results in folds running in the feed direction, which are impressed on the rollers.
In a drying oven, the sheet passes zones conditioned by design and process and, thus, rollers of different temperatures. Rollers at higher temperatures, in comparison with the sheet, suddenly heat the sheet. Thereby, bucklings in the crosswise direction result due to heat expansion; this also results in the so-called heat folds being longitudinally oriented. Soiled or damaged rollers cause deformations of the sheet and must be laborously dismantled, cleaned or exchanged. The resultant interruption of production is an inherent disadvantage. Impairment of the bearing lubrication of the rollers by action of heat and steam in the course of time results in scratches in the sheet because of difficulty-moved or locked rollers.